Laminated steel sheet for two-piece can, method for manufacturing two-piece can, and two-piece laminated can

ABSTRACT

A laminated steel sheet including a polyester resin layer arranged on at least one face of the steel sheet, polyester resin forming the polyester resin layer is obtained by polycondensation of dicarboxylic acid and diol components, the dicarboxylic component contains terephthalic acid as the main ingredient and a copolymerizing ingredient, the diol component comprises ethylene and/or butylene glycol as a main ingredient and a copolymerizing ingredient, the sum of the copolymerizing ingredient in the dicarboxylic acid and the copolymerizing ingredient in the diol component is 8 to 16% by mole in the polyester resin, and the polyester resin layer has 0.06 or less of plane orientation factor.

RELATED APPLICATION

This is a divisional of U.S. application Ser. No. 11/990,371, filed Feb.12, 2008, which is a §371 of International Application No.PCT/JP2006/316122, with an international filing date of Aug. 10, 2006(WO 2007/020951 A1, published Feb. 22, 2007), which is based on JapanesePatent Application No. 2005-234562, filed Aug. 12, 2005.

TECHNICAL FIELD

This disclosure relates to laminated steel sheets suitable formanufacturing two-piece cans of high strain level, such as aerosol cans,methods for manufacturing two-piece cans, and two-piece laminated cansof high strain level.

BACKGROUND

Metal containers of aerosol are largely grouped into two-piece cans andthree-piece cans. The two-piece can is a can structured by two segments,namely the can body integrated with the can bottom and the can end. Thethree-piece can is a can structured by three segments, namely the canbody, the top end, and the bottom end. The two-piece can has no seam(welded part) so that it gives beautiful appearance. However, thetwo-piece can generally requires high strain. Since the three-piece canhas the seam, it is inferior in appearance to the two-piece can. Thethree-piece can, however, generally requires low strain. Therefore, thetwo-piece can is widely used for small capacity and high grade goods inthe market, and the three-piece can is generally used for large capacityand low price goods.

The metal base material for an aerosol two-piece can usually adoptsexpensive and thick aluminum sheet, and rarely uses steel sheet basematerial such as inexpensive and thin sheet, including tinplate andtin-free steel. The reason is that, since the aerosol two-piece canrequires high strain, drawing and DI working are difficult to apply,while aluminum allows applying impact-molding applicable to softmetallic materials. In this situation, if the steel sheet base materialsuch as tinplate and tin-free steel which are inexpensive and highstrength even with a thin sheet thickness is applicable, the industrialsignificance becomes remarkably high.

Although there were many proposals of drawing and DI working methods oflaminated steel sheet, there is no proposal of the method formanufacturing cans such as an aerosol two-piece can of large drawingratio and high elongation in the can height direction.

For example, Examined Japanese Patent Publication No. 7-106394, JapanesePatent No. 2526725 and Japanese Patent Laid-Open No. 2004-148324disclose the working methods for drawing and drawing-ironing forresin-laminated metal sheet. The strain level described in ExaminedJapanese Patent Publication No. 7-106394, Japanese Patent No. 2526725and Japanese Patent Laid-Open No. 2004-148324 (drawing ratio in ExaminedJapanese Patent Publication No. 7-106394, Japanese Patent No. 2526725and Japanese Patent Laid-Open No. 2004-148324), is lower than the rangespecified. This is because Examined Japanese Patent Publication No.7-106394, Japanese Patent No. 2526725 and Japanese Patent Laid-Open No.2004-148324 place the target to beverage cans, food cans, and the like,and beverage cans and food cans are the cans requiring lower strain thanthe range of strain level specified.

Japanese Patent No. 2526725 and Japanese Patent Laid-Open No.2004-148324 describe that, aiming to gain the prevention of delaminationof resin layer and the barrier property after working, a heat treatmentis applied during working and/or at an interim stage of working, or atthe final stage. Japanese Patent No. 2526725 uses an orientatingthermoplastic resin, and Japanese Patent Laid-Open No. 2004-148324 usesa compound of saturated polyester and ionomer.

Examined Japanese Patent Publication Nos. 59-35344 and 61-22626describes methods of relaxing internal stress mainly by applying heattreatment at or above the melting point of the resin, and describe theapplication of heat treatment at a stage after the can-forming. Thestrain level of the can is low suggested by the detail description andby the description of examples.

Japanese Patent No. 2526725 proposes heat treatment in order to relaxthe internal stress and to enhance the orientation crystallization,which method has become common to beverage can and the like. AlthoughJapanese Patent No. 2526725 does not give detail description, thetemperature of heat treatment is presumably at or below the meltingpoint since the orientation crystallization is accelerated at or belowthe melting point. The description and the examples of Japanese PatentNo. 2526725 show that the strain level is lower than the strain levelspecified.

Conventional technologies did not provide methods for manufacturing canssuch as aerosol two-piece cans using laminated steel sheet applying highstrain. Thus, we fabricated two-piece cans using laminated steel sheetapplying high strain of the steps of drawing-ironing of the laminatedsteel sheet to form into a shape of a cylinder integrated with a bottom,followed by diametral reduction in the vicinity of opening of thecylinder, and found the occurrence of problems characteristic to highstrain, specifically the problem of delamination and fracture of resinlayer. Our efforts revealed the effectiveness of the heat treatment inqualitative view. However, sole heat treatment was not sufficient, andthe delamination of resin layer unavoidably appeared in a zone of highstrain. As a result, simple application of the related art did not solvethe problem of delamination of the resin layer. In addition, thereappeared a problem of deterioration of formability of the resin layerduring the forming after the heat treatment.

It could therefore be advantageous to provide a laminated steel sheetfor a two-piece can which prevents delamination and fracture of thelaminate resin layer even when a can of high strain level such as anaerosol two-piece can is manufactured, and to provide a method formanufacturing the two-piece can. It could also be advantageous toprovide a can of high strain level, such as an aerosol two-piece can,using the laminated steel sheet.

SUMMARY

An important characteristic required for the resin layer during the highstrain forming is the difficulty in orientation. Resin usually orientsin the can height direction owing to the compressive deformation in thecircumferential direction and to the elongational deformation in thecan-height direction. Our investigations revealed that polyethyleneterephthalate copolymers or polybutylene terephthalate copolymers arepromising ones for above high strain forming. Our investigations alsofound that, for polyethylene terephthalate copolymers or polybutyleneterephthalate copolymers, a smaller quantity of copolymerizationcomponent causes generation of more cracks (fractures) in parallel witheach other in the can-height direction. We also found that orientationsimilarly progressed even for the case of applying heat treatment to thecan after working Then, we found that increasing the quantity of thecopolymerization component of resin solves the above problems.

We thus provide the following:

(1) A laminated steel sheet used for producing a two-piece can, whichsatisfies the following relations:

0.1≦d/R≦0.25

1.5≦h/(R−r)≦4

wherein

-   -   h is a height of a final formed body,    -   r is a maximum radius of the final formed body,    -   d is a minimum radius of the final formed body (including the        case that r and d are equal),    -   R is a radius of a circular disk, before forming, having the        same weight to that of the final formed body,        wherein    -   at least one face of the steel sheet has a polyester resin        layer,    -   the polyester resin is obtained by polycondensation of a        dicarboxylic acid component and a diol component,    -   the dicarboxylic component contains terephthalic acid as the        main ingredient and a copolymerizing ingredient,    -   the diol component contains ethylene glycol and/or butylene        glycol as the main ingredient and a copolymerizing ingredient,        the sum of the copolymerizing ingredient in the dicarboxylic        acid and the copolymerizing ingredient in the diol component is        ranging from 8 to 16% by mole in the polyester resin, and    -   the polyester resin laminate layer has 0.06 or less of plane        orientation factor.

(2) The laminated steel sheet for a two-piece can according to (1),wherein

-   -   the dicarboxylic acid component contains an isophthalic acid        ingredient in the copolymerization ingredient, and    -   the diol component contains diethylene glycol and/or cyclohexane        diol in the copolymerizing ingredient.

(3) A method for manufacturing a two-piece can, comprising the step ofmulti-stage forming of a circular disk of laminated steel sheet into afinal formed body which satisfies the following relations:

0.1≦d/R≦0.25

1.5≦h/(R−r)≦4

wherein

-   -   h is a height of a final formed body,    -   r is a maximum radius of the final formed body,    -   d is a minimum radius of the final formed body (including the        case that r and d are equal),    -   R is a radius of a circular disk, before forming, having the        same weight to that of the final formed body,        wherein the laminated steel sheet is used as the laminated steel        sheet.

(4) A method for manufacturing a two-piece can, comprising the step ofproducing the final formed body according to (3), wherein heat treatmentis applied as an interim stage of forming so as the formed body is to beheated to a temperature of from 150° C. to the melting point of thepolyester resin.

(5) The method for manufacturing a two-piece can according to (4),wherein the heat treatment is carried out two or more times during theforming stage.

(6) The method for manufacturing a two-piece can according to (4),wherein the heat treatment is applied at an interim stage that satisfiesthe following relations:

0.1≦d/R≦0.25

1.5≦h/(R−r)≦4

wherein

-   -   h is a height of the formed body in the interim stage,    -   r is a maximum radius of the formed body in the interim stage,    -   d is a minimum radius d of the formed body in the interim stage        (including the case that r and d are equal),    -   R is a radius R of the circular disk, before forming, having the        same weight to that of the final formed body.

(7) A two-piece laminated can manufactured by the method according toany of (3) to (6).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an example of the manufacturing process of a can.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of the manufacturing process of the can,giving the order of steps of forming a circular blank into a formed bodyin a shape of a cylinder integrated with the bottom by drawing(including DI forming), and of applying diametral reduction in thevicinity of opening of the formed body, thus obtaining a two-piece canwith a diametral reduction part in the vicinity of the opening.

In FIG. 1, reference symbol 1 is the circular disk blank (blank sheet)before forming, 2 is the straight wall part as the base part of theformed body, (in the step D, straight wall part not being worked bydiametral reduction), 3 is the dome-shaped part, 4 is the straight wallpart at the neck-shaped portion being worked by diametral reduction, and5 is the taper-shape part, or the tapered wall part after worked bydiametral reduction.

First, the circular disk blank 1 is subjected to one or a plurality ofsteps of drawing (including DI forming) to form a formed body in a shapeof a cylinder integrated with a bottom, having a specified can diameter(radius r: radius of outer face of can), (Step A). Then, the bottom partof the formed body is subjected to dome-forming, or to forming into anupward convex shape to form the dome-shaped part 3, (Step B). Furtherthe edge of the opening of the formed body is trimmed, (Step C). Next,the opening of the formed body is subjected to one or a plurality ofstages of diametral reduction to bring the opening side of the formedbody to a specified can diameter (radius d: radius of the can outerface), thus obtaining the desired final formed body (two-piece can). InFIG. 1, the reference symbol R₀ is the radius of the circular disk blank1 before forming, and h, r, and d are the height, the maximum radius,and the minimum radius of the formed body during forming or of the finalformed body, respectively, and R is the radius of the circular disk,before forming, having equal weight to that of the final formed body.According to the manufacturing process of the two-piece can, Step Aresults in the maximum radius equal to the minimum radius, or r=d, whileStep D results in r>d.

The radius R of the circular disk, before forming, having the sameweight as that of the final formed body is determined based on themeasured weight of the final formed body. That is, the weight of thefinal formed body is measured, and the size (radius) of the circulardisk, before forming, having the same weight as the measured weight isdetermined, which determined size is used as the radius R of thecircular disk, before forming, having the same weight as that of thefinal formed body. The can edge portion is trimmed during the canmanufacturing process. Since, however, the radius R of the circulardisk, before forming, having the same weight as that of the final formedbody eliminates the effect of the trimming, a more suitable evaluationof the strain is available.

On the two-piece can which is fabricated by the above drawing (includingDI working) and diametral reduction applied to the circular disk blank,the resin layer is elongated in the height direction and compressed inthe circumferential direction. When the strain level is high,deformation of the resin becomes large, which leads to fracture of theresin layer. The index of strain level is not only the parameter d/Rrepresenting the degree of compression, but also the parameter [h/(R−r)]relating to the elongation in the can height direction because theexpression of strain level in a high strain zone needs to considerelongation in addition to the drawing ratio. That is, by specifying thestrain level by both the degree of compression and the degree ofelongation, the degree of deformation of the resin layer is quantified.By elongation in the height direction and compression in thecircumferential direction, the resin layer tends to delaminate, thus,elongation in the height direction becomes an important variable addingto the degree of compression.

The strain level of the final manufactured can (final formed body) isspecified such that the relation of the height h of the final formedbody, the maximum radius r thereof, the minimum radius d thereof, andthe radius R of the circular disk, before forming, has the same weightas that of the final formed body, to satisfy [0.1≦d/R≦0.25] and[1.5≦h/(R−r)≦4].

Using conventional technologies, it has been difficult to manufacture ahigh strain level of can that satisfies both the parameter d/Rspecifying the degree of compression not higher than 0.25 and theparameter [h/(R−r)] specifying the degree of elongation not smaller than1.5 using a laminated steel sheet. Consequently, we specify the strainlevel d/R of the manufacturing can as 0.25 or less, and [h/(R−r)] as 1.5or more.

If the strain level is high enough to result in the parameter d/Rspecifying the degree of compression not higher than 1.0 or theparameter [h/(R−r)] specifying the degree of elongation exceeding 4, thenumber of forming stages increases even if forming is available, or thesheet elongation reaches its limit by the progress of work hardening,which causes the sheet fracture problem. Therefore, we specify thestrain level of manufacturing a can as [0.1≦d/R] and [h/(R−r)≦4].

The multiple stage forming may be any of drawing, drawing-ironing,diametral reduction, and combinations thereof. If the diametralreduction is included in working, the size d of the final formed body is[r>d]. If the diametral reduction is not included, the size of the finalformed body is [r=d] (r and d are the radius of final formed body).

We specify the laminated steel sheet with a resin laminate as the metalsheet of base material.

Steel sheet is selected as the base metallic material because steel isless expensive and superior in economy to aluminum. The steel sheet canbe ordinary tin-free steel or tinplate. Tin-free steel preferably has ametal chromium layer of about 50 to about 200 mg/m² of surface coatingweight and a chromium oxide layer of about 3 to about 30 mg/m² ofcoating weight as metal chromium. Tinplate preferably has about 0.5 toabout 15 g/m² of plating. The sheet thickness is not specificallylimited, and that in a range from about 0.15 to about 0.30 mm, forexample, is applicable. If no economic consideration is needed, thetechnology can simply apply also to aluminum base material.

An important characteristic required for the resin layer during theabove-described high strain forming is the difficulty in orientation.Resin usually orients in the can height direction owing to compressivedeformation in the circumferential direction and to elongationaldeformation in the can-height direction. Our investigations foundpromising resins for the high strain forming in view of difficulty inorientation, which resins are prepared by polycondensation of adicarboxylic acid component with a diol component, wherein thedicarboxylic acid component has terephthalic acid as the main ingredientand contains or does not contain isophthalic acid as anothercopolymerization ingredient, and the diol component has ethylene glycoland/or butylene glycol as the main ingredient and contains or does notcontain diethylene glycol and cyclohexane diol as anothercopolymerization ingredient. Consequently, we specify the kind of theresin in the resin layer to above resins.

Our investigations, however, also found that, for the polyethyleneterephthalate copolymers or polybutylene terephthalate copolymers, themolecules tend to become orientated when the percentage of thecopolymerization ingredient is low. That is, a smaller quantity ofcopolymerization component is likely to generate more cracks (fractures)in parallel with each other in the can-height direction in a zone ofhigh strain level. In concrete terms, it was found that, when theorientation in the resin in the can height direction becomessignificant, the bonding force in the circumferential direction becomesweak, which results in fractures in the circumferential direction. Theorientation similarly progresses even in the case of applying heattreatment to the can after working To prevent these problems, the lowerlimit of the percentage of the copolymerization ingredient is 8% bymole. From the point of difficulty in orientation, a higher percentageof copolymerization ingredient is preferred. However, a percentage above16% by mole increases the film cost and deteriorates economy. From thispoint of view, the upper limit of the copolymerization percentage is 16%by mole. Therefore, if cost is not considered, high copolymerizationpercentage is also suitable.

For the resin layer to follow the deformation accompanying the highstrain level, we found that the orientation of laminated steel sheet inthe initial stage of working is also an important variable. A filmprepared by biaxial orientation or the like is oriented in the planedirection. However, if the film is kept in a high orientation stateafter lamination, the film cannot follow the working and results infractures. From this point of view, the plane-orientation factor shouldbe 0.06 or less.

For preparing the laminated steel sheet using a biaxially oriented filmwith high plane-orientation factor, the temperature during lamination isincreased to fully melt the oriented crystals. A film which is preparedby extrusion is suitable because the film is oriented very little.Similarly, the direct lamination method which directly laminates amolten resin on the steel sheet is also suitable because of the samereason.

Upon fabricating the can of high strain level, delamination of laminatemay occur depending on the working conditions and the kind of resin. Insuch a case, it is effective that one or more heat treatments is appliedat an interim stage of the multiple stage forming, by heating the formedbody to a temperature ranging from 150° C. to the melting point of thepolyester resin. Although the heat treatment is performed to relax theinternal stress generated by working, relaxation of internal stressaffects the recovery of adhesion. That is, the can of high strain levelcauses large degree of strain in the resin layer, thus likely inducinglarge internal stress. As a result, the delamination of resin layer mayoccur with the internal stress as the driving force. Application ofadequate heat treatment in a stage of high internal stress beforegenerating delamination, during the forming process, relaxes theinternal stress to prevent delamination. The heat treatment, however,has a drawback of progress of orientation to crystallize the resin, thusto deteriorate the formability of the resin layer. In particular, in azone of high strain level, there may be a need for working after heattreatment. Since working after the heat treatment causes the resin toeasily fracture owing to the orientation crystallization, theorientation crystallization is harmful.

From this point of view, we specify the conditions and timing of theheat treatment, adding to the limitation of the kind of resin.

The condition of heat treatment to apply in an interim stage to heat theformed body to a temperature should be in a range from 150° C. to themelting point of the polyester resin. As described above, by selecting aresin that is difficult to orient, orientation crystallization duringheat treatment can be suppressed, and the lower limit of thecopolymerization percentage is specified.

If the heat treatment temperature exceeds the melting point of thepolyester resin, there appear harmful phenomena such as a rough surfacelayer and adhesion of resin to other material. On the other hand, thelower limit of the heat treatment is takes into account the efficiencyof relaxation of the internal stress. That is, although relaxation ofinternal stress proceeds at a temperature at or above the glasstransition point of the polyester resin, excessively low temperaturestake a time for relaxation of the stress. Thus, the lower limit of theheat treatment should be 150° C. Therefore, in a manufacturing processwhich does not care about the treatment time, the treatment temperatureof 150° C. or below can be adopted. Generally, however, a long time oftreatment deteriorates productivity. Preferable conditions of heattreatment are between 170° C. and [the melting point of polyesterresin]-20° C.

The heat treatment in an interim stage which satisfies the relation ofthe height h of formed body at the interim stage, the maximum radius rthereof, the minimum radius d (including the case that r and d areequal) thereof, with the radius R of the circular disk, before forming,having the same weight to that of the final formed body, is[0.2≦d/R≦0.5] and [1.5≦h/(R−r)≦2.5].

This is because the most effective heat treatment can be provided whenthe strain level is in that range. That is, the heat treatment in astage of mild strain level gives only a small effect because relaxationof internal stress is conducted at a stage of not-high internal stress.Heat treatment at a stage of excessively high strain level deterioratesadhesion, thus delamination may occur, which timing is too late. Fromthis point of view, the upper limit and the lower limit of the strainlevel are specified as above.

The method of heat treatment is not specifically limited, and it wasconfirmed that electric oven, gas oven, infrared furnace, and inductionheater result in similar effects. Heating rate, heating time, andcooling time (the time between the completion of heat treatment and thecooling to the glass transition point of resin) are adequately selectedconsidering the positive effect of relaxing internal stress and thenegative effect of orientation crystallization. A higher heating rate ismore efficient, and an adequate range of heating time is from about 15seconds to about 60 seconds. The heating time is, however, not limitedto the range. A long cooling time is not preferred in terms of qualitybecause the amount of generated spherulites increases. Consequently, ashorter cooling time is better.

The laminated steel sheet may contain additives such as pigment,lubricant, and stabilizer in the resin layer, and adding to the resinlayer a resin layer having other functions may be located at an upperlayer or at an intermediate layer above the steel sheet.

Although a thinner resin layer deteriorates formability, the resin layerprovides good formability even as a thin layer. The thickness of theresin layer is adequately selected depending on the strain level andother required characteristics. For example, the thickness thereofbetween 5 to 50 μm is suitably applied. In particular, a thin resinlayer of 20 μm or less is a zone of high contribution.

The laminated steel sheet is required to have a laminate of resin layeron at least one side of the steel sheet.

The lamination method for the steel sheet is not specifically limited,and the method is adequately selected such as the heat lamination methodwhich thermally bonds a biaxially oriented film or non-oriented film,and the extrusion method which directly forms a resin layer on the steelsheet using T-die and the like. Both of the methods were confirmed togive sufficient effect.

EXAMPLE 1

Selected examples are described below.

[Preparation of Laminated Steel Sheet]

The substrate metal sheet was 0.20 mm thick T4CA, TFS (120 mg/m² ofmetal Cr layer, 10 mg/m² of chromium oxide layer as metal Cr). Onto theoriginal sheet, various kinds of resin layers were formed using thefilm-lamination method (heat lamination method) or the direct-laminationmethod (direct extrusion method). For the film-lamination method, twokinds of the methods were applied, using a biaxially oriented film and anon-oriented film. On both sides of the metal sheet, each film having 20μm in thickness was laminated.

The plane orientation factor of the laminate film on thus preparedlaminated steel sheet was determined by the following procedure.

[Determination of Plane Orientation Factor]

Abbe's refractometer was used to determine the refractive index underthe condition of: light source of sodium/D ray; intermediate liquid ofmethylene iodide; and temperature of 25° C. The determined refractiveindexes were Nx in the machine direction, Ny in the transversedirection, and Nz in the thickness direction of the film. Then, theplane orientation factor Ns was calculated by the following formula:

Plane orientation factor (Ns)=(Nx+Ny)/2−Nz.

TABLE 1 shows the manufacturing method and the characteristics of thelaminated steel sheet.

The lamination methods are the following:

Heat lamination method 1: A film prepared by the biaxial orientationmethod was thermocompressed on a steel sheet which was heated to [themelting point of resin+10° C.] using a nip roll. Then the film wascooled within 7 seconds by water.

Heat lamination method 2: A non-oriented film was thermocompressed on asteel sheet which was heated to [the melting point of resin+10° C.]using a nip roll. Then the film was cooled within 7 seconds by water.

Direct extrusion method: Resin pellets were kneaded and melted in anextruder, which were then extruded through a T-die to laminate onto arunning steel sheet. The steel sheet with the resin laminate wasnip-cooled on a cooling roll at 80° C., and was further cooled by water.

TABLE 1 Tested Percentage of steel copolymerization ingredient MeltingPlane orientation sheet No. Kind of resin (mol %) point (° C.)Lamination method factor Remark A1 PET-I 8 238 Heat lamination method 10.02 Steel sheet of the invention A2 PET-I 10 233 Heat lamination method1 0.02 Steel sheet of the invention A3 PET-I 12 228 Heat laminationmethod 1 0.02 Steel sheet of the invention A4 PET-I 14 223 Heatlamination method 1 0.02 Steel sheet of the invention A5 PET-I 16 218Heat lamination method 1 0.02 Steel sheet of the invention A6 PET-I 12228 Heat lamination method 1 0.06 Steel sheet of the invention A7 PET-I12 228 Heat lamination method 1 0.04 Steel sheet of the invention A8PET-I 12 228 Heat lamination method 1 0.01 Steel sheet of the inventionA9 PET-I 12 228 Heat lamination method 1 <0.01 Steel sheet of theinvention A10 PET-I 12 228 Heat lamination method 2 <0.01 Steel sheet ofthe invention A11 PET-I 12 228 Direct extrusion method <0.01 Steel sheetof the invention A12 PET-I 12 202 Heat lamination method 2 <0.01 Steelsheet of the invention A13 PET-PBT(60)-I 12 211 Heat lamination method 2<0.01 Steel sheet of the invention A14 PET-DEG 12 225 Heat laminationmethod 2 <0.01 Steel sheet of the invention A15 PET-CHDM 12 224 Heatlamination method 2 <0.01 Steel sheet of the invention A16 PET-I 4 248Heat lamination method 1 0.02 Steel sheet of Comparative Example A17PET-I 2 253 Heat lamination method 1 0.02 Steel sheet of ComparativeExample A18 PET-I 12 228 Heat lamination method 1 0.08 Steel sheet ofComparative Example A19 PET 0 258 Heat lamination method 1 0.02 Steelsheet of Comparative Example PET: Polyethylene terephthalate PET-I:Copolymer of polyethylene terephthalate and isophthalate (isophthalicacid copolymerization percentage: 12 mol %) PET-PBT(60)-I: Copolymer ofpolyethylene terephthalate, butylene terephthalate, and isophthalate(Butylene terephthalate copolymerization percentage: 60 mol %,isophthalic acid copolymerization percentage: 12 mol %) PET-DEG:Copolymer of terephthalic acid, ethylene glycol, and diethylene glycolPET-CHDM: Copolymer of terephthalic acid, ethylene glycol, andcyclohexane dimethanol

[Can Forming]

Using the prepared testing steel sheet, the can (final formed body) wasmanufactured in accordance with the manufacturing steps shown in FIG. 1,following the procedure described below. TABLE 2 shows the shape of theintermediate formed body (Step C) and the final formed body (Step D).The drawing in Step A was given by five stages, while the diametralreduction in Step D was given by seven stages. The heat treatment wasgiven at a stage between Step A and Step D, heating the can using aninfrared heating furnace. After the heat treatment, the can was cooledby water. TABLE 3 shows the timing of heat treatment (strain level ofthe can when the heat treatment is performed) and the condition of heattreatment.

In TABLE 2, the reference symbols h, r, d, ha, hc, and R of the finalformed body (Step D) are the height of the final formed body up to theopening end, the radius of the base part 2, the radius of theneck-shaped part 3, the height of the base part 2, the height of theneck-shaped part 3, and the radius of the circular disk blank, beforeforming, having the same weight to that of the final formed body,respectively, (refer to FIG. 1). The radius R of the circular disk blankwas determined by the following procedure. The weight of the blank sheetbefore forming and the weight of the final formed body after the step oftrimming were measured. Based on thus measured weights, the radius ofthe blank sheet, before forming, having the same weight to that of thefinal formed body was determined. Thus determined radius was adopted asthe radius R of the circular disk blank, before forming, having the sameweight to that of the final formed body.

TABLE 2 Intermediate formed Final formed Blank body (Step C) body (StepD) Change rate Can diameter r h r d h ha hc Blank diameter of sheetshape R_(O) (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm) R * (mm) d/R h/(R −r) thickness ** B1 41.0 11.0 63.6 11.0 7.8 65.9 47.0 9.9 40.4 0.19 2.241.20 B2 47.0 11.0 63.5 11.0 7.8 65.9 47.0 9.9 46.6 0.17 1.85 1.45 B335.5 11.0 63.5 11.0 7.8 65.9 47.0 9.9 34.8 0.22 2.77 0.75 B4 33.0 11.063.5 11.0 7.8 65.9 47.0 9.9 32.2 0.24 3.11 0.65 * The blank diameter Ris derived from conversion of the weight of the final formed body. **Change rate of sheet thickness = Sheet thickness at the thinnest portionof the can/Thickness of the blank sheet. Both are the steel sheetthickness.

Testing Strain level at Condition of heat treatment Can steel Meltingpoint heat treatment Temperature Time Final can Film Film No. sheet No.of resin (° C.) d/R h/(R − r) (° C.) (sec.) shape formability adhesionRemark C1 A3 228 0.38 1.78 220 30 B1 ◯ ⊚ Example C2 A3 228 0.38 1.78 22060 B1 ◯ ⊚ Example C3 A3 228 0.38 1.78 220 90 B1 ◯ ⊚ Example C4 A3 2280.38 1.78 220 120 B1 ◯ ⊚ Example C5 A3 228 0.38 1.78 228 30 B1 ◯ ⊚Example C6 A3 228 0.38 1.78 160 80 B1 ◯ ⊚ Example C7 A3 228 0.38 1.78180 60 B1 ◯ ⊚ Example C8 A3 228 0.38 1.78 200 45 B1 ◯ ⊚ Example C9 A3228 0.38 1.78 220 30 B1 ◯ ⊚ Example C10 A3 228 0.30 2.10 220 30 B1 ◯ ⊚Example C11 A3 228 0.47 1.53 220 30 B1 ◯ ⊚ Example C12 A3 228 0.28 1.80220 30 B2 ◯ ⊚ Example C13 A3 228 0.31 1.70 220 30 B2 ◯ ⊚ Example C14 A3228 0.50 2.30 220 30 B3 ◯ ⊚ Example C15 A3 228 0.23 2.70 220 30 B3 ◯ ⊚Example C16 A3 228 0.55 0.20 220 30 B3 ◯ ◯ Example C17 A3 228 0.40 2.80220 30 B4 ◯ ◯ Example C18 A3 228 0.35 2.90 220 30 B4 ◯ ◯ Example C19 A3228 0.23 3.05 220 30 B4 ◯ ◯ Example C20 A3 228 Without heat treatment B3◯ ◯ Example C21 A3 228 0.38 1.78 220 30 B1 ◯ ⊚ Example C22 A3 228 0.381.78 220 30 B1 ◯ ⊚ Example C23 A1 238 0.38 1.78 230 30 B1 ◯ ⊚ ExampleC24 A2 233 0.38 1.78 230 30 B1 ◯ ⊚ Example C25 A4 223 0.38 1.78 215 30B1 ◯ ⊚ Example C26 A5 218 0.38 1.78 210 30 B1 ◯ ⊚ Example C27 A6 2280.38 1.78 220 30 B1 ◯ ⊚ Example C28 A7 228 0.38 1.78 220 30 B1 ◯ ⊚Example C29 A8 228 0.38 1.78 220 30 B1 ◯ ⊚ Example C30 A9 228 0.38 1.78220 30 B1 ◯ ⊚ Example C31 A10 228 0.38 1.78 220 30 B1 ◯ ⊚ Example C32A11 228 0.38 1.78 220 30 B1 ◯ ⊚ Example C33 A12 202 0.38 1.78 220 30 B1◯ ⊚ Example C34 A13 211 0.38 1.78 205 30 B1 ◯ ⊚ Example C35 A14 225 0.381.78 220 30 B1 ◯ ⊚ Example C36 A15 224 0.38 1.78 220 30 B1 ◯ ⊚ ExampleC37 A16 248 0.38 1.78 240 30 B1 X ⊚ Comparative Example C38 A17 253 0.381.78 245 30 B1 X ⊚ Comparative Example C39 A18 228 0.38 1.78 220 30 B1 X◯ Comparative Example C40 A19 258 0.38 1.78 250 30 B1 X ⊚ ComparativeExample

1) Blanking (66 to 94 mmφ)

2) Drawing and Ironing (Step A)

Through the five stages of drawing, the cans (intermediate formedbodies) having radius r and height h of the can in a range of r/R from0.27 to 0.34 and of [h/(R−r)] from 1.84 to 3.09, were manufactured. Tomanufacture the desired cans, ironing was also applied at need.

3) Forming of Dome-Shape at Can Bottom (Step B)

Stretching was applied to the can bottom to a hemispherical shape of 6mm in depth.

4) Trimming (Step C)

The can top edge portion was trimmed by about 2 mm.

5) Diametral Reduction at Opening Portion of the Cylinder (Step D)

Diametral reduction was given to the opening portion of the cylinder. Inconcrete terms, the diametral reduction was conducted by the die-neckmethod in which the opening end was pressed against a die in aninside-tapered shape, thus manufactured the cans having final can shapegiven in TABLE 2.

For the cans manufactured by the above procedure, evaluation was givenin terms of the adhesion of the film layer to the can, the formabilityof the film layer, and the appearance of the film layer. The results ofthe evaluation are also given in TABLE 3.

[Adhesion Test]

The can was sheared in approximately rectangular shape in the can heightdirection, having 15 mm of width in the circumferential direction. Withthus sheared piece, only the steel sheet was sheared at a position of 10mm from the bottom in the can height direction, straight in thecircumferential direction. As a result, there was prepared a test piecehaving a 10 mm portion in the can height direction toward the can bottomand a residual portion with the boundary of the sheared position. At the10 mm portion, a steel sheet having 15 mm in width and 60 mm in lengthwas joined (welded). Then, the 60 mm steel sheet portion was clamped toforcefully separate the film on the residual portion by about 10 mm fromthe sheared position. The peeling test was conducted in 180° directionwith the clamping areas of the film-separated portion and the 60 mmsteel sheet portion. The minimum peeling strength among the observedvalues was adopted as the index of adhesion.

[Peeling Strength]

Less than 4N/15 mm: X

4N/15 mm or more and less than 6N/15 mm ∘

6N/15 mm or more: ⊚

[Evaluation of Film Formability]

The outer surface of the resin layer after the can forming was observedvisually and with a light microscope to confirm the presence/absence offracture of film. The resin layer giving normal appearance was evaluatedas ∘, and the resin layer showing fracture and crack was evaluated as X.

[Results of Evaluation]

Cans C1 to C15 and C21 to C36 are Examples, and were subjected to heattreatment within our range. They showed good film formability andadhesion.

Cans C16 to C20 are Examples. However, they were not subjected to apreferred heat treatment, or they were subjected to the heat treatmentoutside the preferred timing. As a result, they gave only ∘ evaluationfor the adhesion, though both the film formability and the adhesionpassed the evaluation.

Cans C37, C38, and C40 are Comparative Examples. The evaluation offormability was X because the copolymerization percentage of isophthalicacid was outside our range.

Can C39 is an example having a plane orientation factor outside ourrange, and the formability was evaluated to X.

1. A method for manufacturing a two-piece can comprising multi-stageforming a circular disk of the laminated steel sheet comprising: apolyester resin layer arranged on at least one face of the steel sheet,polyester resin forming the polyester resin layer is obtained bypolycondensation of a dicarboxylic acid component and a diol component,the dicarboxylic component contains terephthalic acid as the mainingredient and a copolymerizing ingredient, the diol component comprisesethylene glycol and/or butylene glycol as a main ingredient and acopolymerizing ingredient, the sum of the copolymerizing ingredient inthe dicarboxylic acid and the copolymerizing ingredient in the diolcomponent ranges from 8 to 16% by mole in the polyester resin, and thepolyester resin layer has 0.06 or less of plane orientation factor intoa final formed body which satisfies the following relations:0.1≦d/R≦0.251.5≦h/(R−r)≦4 wherein h is a height of a final formed body, r is amaximum radius of the final formed body, d is a minimum radius of thefinal formed body (including when r and d are equal), R is a radius of acircular disk, before forming, having the same weight as that of thefinal formed body.
 2. A method for manufacturing a two-piece cancomprising producing the final formed body according to claim 1, whereinheat treatment is applied as an interim stage of forming so that theformed body is heated to a temperature of from 150° C. to the meltingpoint of the polyester resin.
 3. The method according to claim 2,wherein the heat treatment is carried out two or more times duringforming.
 4. The method according to claim 2, wherein the heat treatmentis applied at an interim stage that satisfies the following relations:0.1≦d/R≦0.251.5≦h/(R−r)≦4 wherein h is a height of the formed body in the interimstage, r is a maximum radius of the formed body in the interim stage, dis a minimum radius d of the formed body in the interim stage (includingwhen r and d are equal), R is a radius R of the circular disk, beforeforming, having the same weight as that of the final formed body.